Despite the best efforts to date, 46 million people still smoke, and 430,000 people die from tobacco-related illness annually. The question remains why some people smoke and not others. The proposed research plan seeks to explore, for the first time, the genetic influences on the disposition and kinetics of nicotine, stable characteristics of individuals that may be central to the understanding of nicotine addiction and its effective treatment. We will recruit 250 twin pairs over 4 years (100 monozygotic [MZ] and 100 dizygotic [DZ] never-smoking pairs, 25 MZ and 25 DZ pairs in which one twin is a never smoker an the other twin is a current or former smoker). The inclusion of never-smoking twins will allow us to test initial biological differences and similarities in nicotine metabolism and kinetics in a group with no previous exposure to nicotine. The inclusion of MZ and DZ twin pairs discordant for ever smoking will facilitate the estimation of the genetic correlation between nicotine metabolism and nicotine dependence. Reported zygosity and smoking status will be confirmed by biochemical methods. After completion of a brief questionnaire designed to screen for exclusionary health conditions and to assess potentially confounding variables, subjects will be seen as twin pairs for an 8-hour in-hospital nicotine infusion test. During the 8 hours after the 30-minute infusion, blood and urine samples will be taken at regular intervals for assay of nicotine and metabolites. At each of the subsequent four mornings, project staff will drive to the homes of the subjects to collect blood and 24-hr urine samples. After the assay is completed, the data will be subjected to both univariate and multivariate genetic analyses to determine genetic and environmental influences on each pharmacokinetic parameter individually and in combination with other biologically meaningful indices of nicotine metabolism. Implications of this project are seen for: 1) estimation of the impact genetic influences on nicotine metabolism and 2) further refinement of the metabolic phenotype associated most strongly with the genetic predisposition to nicotine addiction. Important future directions lie in combining knowledge from this study with that from the companion applications in the University of Michigan Nicotine Research Consortium, including: 1) genetic influences on nicotine metabolism and metabolism and sensitivity and 2) shared genetic influences with cofactors such as Adult Attention Deficit Disorder and nicotine metabolism. Through later molecular analysis of blood to be collected as part of this project, we may be able to identify gene variants that underlie the amount and function of various enzymes responsible for nicotine metabolism. Ultimately, this knowledge may lead to more informed and more finely targeted treatment of individuals who are either susceptible to nicotine addiction or already addicted.